spec: record why the try rule cannot be enforced yet

SPEC.md allows `try` only inside a function returning an error union, but
check.c only tests it in the FE_N_EXPR_STMT case, so `var x = try e;` and
`x = try e;` walk straight past. m5/owned.fe and m5/runtime.fe both depend on
that gap.

Moving the check onto the try expression is a four-line change and it is
correct, but it cannot land yet. runtime.fe's `run` allocates and returns a
value, so closing the hole forces it to return an error union -- and master
rejects `return <value>;` in `-> !i32` ("return type mismatch") as well as a
bare `return;` in `-> !void` ("void expression returned from value function").
Both need contextual success construction, which is M7 work. `catch` and
`@trap()`, the two spellings SPEC offers as alternatives, are also M7-only, so
there is no way to express `run` legally on master today. All three paths were
tried in DOSBox-X, not assumed.

Fix owned.fe now, since `main() -> !void` is legal today and matches
m4/try-fpr.fe, and leave the checker alone until M7 lands with the rest.

Verified: 155 passed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BScg8CF1sAAM2zVHAu5zvW

fec/tests/m5/owned.fe

@@ -1,6 +1,6 @@
 unit m5_owned;
 
-fn main() -> void {
+fn main() -> !void {
     var p: ^i32 = try mem.create(0);
     p = try mem.create(0);
     p.^ = 7;